It is known that a sample of suitable material, bombarded by electrons, returns part of the received energy by photon emission. By collecting said photons by a suitable detector it is possible to build up an image of the analyzed area, thus obtaining information on the material properties. When making measurements by that technique it is necessary to collect and send to the detector as many photons as possible. The simplest solution would be to place the detector as close as possible to the sample emitting surface, but under these conditions the surface of the conventional glass screens protecting the detectors becomes charged by backscattered electrons. This charging generates an electric field which disturbs electron beam scanning. If the detector is placed relatively far from the sample, beyond the reach of the electrons, the signal emitted from the detector can be too low to permit a precise interpretation. Whatever the detector position, mirrors can be used to increase the collection solid angle, but such mirrors generally prevent the microscope use at low magnifications and are difficult to use (ie. constitute an encumbrance in the analysis chamber, make it necessary to center the sample with respect to the mirror, etc.).
The problem of simultaneously ensuring a good collection efficiency and good screening against backscattered electrons can be solved by placing the detector near the sample and placing therebetween a screen which is transparent to photons and thick enough to retain the electrons, which does not give rise to luminescence when struck by backscattered electrons and, finally, which does not give rise to electron charging of its surface, to avoid creating an electrical field capable of disturbing the scanning beam. To meet these requirements, it has been proposed to make a glass screen coated with a very thin metal conductive coating. This solution is described by J. Marek, R. Geiss, L. M. Glassman and M. P. Scott in the paper entitled "A Novel Scheme for Detection of Defects in III-V Semiconductors by Cathodoluminescence", Glass Technology, Vol. 24, No. 3, June 1983.
This known solution has a number of disadvantages. The metal coating increases absorption, consequently reducing the level of the detector output signal, what renders the measurement more sensitive to noise and requires more powerful, and hence more expensive, amplification systems. Additional work is required to fabricate the coating, and this also increases the costs. Finally, conventional oxide glasses present a fair transparency in a spectral region which, in the infrared, does not extend beyond 2.4 .mu.m.